This invention relates generally to molds, and more particularly, to an apparatus and method for molding cores for use in casting hollow parts.
Turbine systems often include a plurality of airfoils, e.g., vanes, nozzles, blades, buckets, which are hollow to provide a cooling passage. An airfoil is typically formed by an investment casting process using a ceramic core. The ceramic core is then leached out leaving a hollow passage in the airfoil.
The ceramic cores themselves are typically molded using metal molds or dies. To remove the ceramic core from the mold without damage, often a preliminary firing is required to impart sufficient strength to the core so that the core may be handled. For example, one portion or half of the mold is removed to expose an outer surface portion of the core. This exposed outer surface portion is then heated, for example, with a torch. Thereafter, the core is removed, placed in an oven, and heated.
A drawback with using metal molds to mold the ceramic cores is that the metal molds are time consuming and expensive to produce. In addition, with a new hollow part, the time and cost to develop the part increases where many design iterations of the part are involved and each design iteration requires fabrication of a new mold for molding the core.
Stereolithography is a rapid prototyping and tooling process that has become widely popular for use in rapidly producing three-dimensional solid objects directly from electronic models. For example, stereolithography has been used to produce molds for molding plastic parts. The process involves developing a solid model from a liquid photopolymer epoxy resin by exposing it to an ultraviolet laser. The resin solidifies in layers about 0.003 inch to about 0.010 inch thick with each pass of the laser.
The use of plastic tooling or molds produced by stereolithography for making ceramic cores is limited due to the requirement of a preliminary firing or torching to impart sufficient strength to the ceramic core so that the ceramic core may be handled. For example, localized temperatures experienced by the mold can be close to about 2,000 degrees Fahrenheit during this preliminary firing or torching, whereas the plastic molding is operable up to temperatures of about 400 degrees Fahrenheit. In particular, the extreme temperatures of the preliminary firing or torching results in unacceptable degradation and distortion of the plastic molds. Accordingly, there is a need in the art for an improved apparatus and method for molding a ceramic core.